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Abstract

The use of stable isotope analysis is becoming more common in all areas of animal ecology. In particular, natural variation in the abundance of stable isotopes makes them useful in studies of foraging ecology. For example, nitrogen stable isotopes can offer insight into trophic relationships and carbon stable isotopes tend to trace the original source of carbon in a system (e.g. - photosynthetic pathway). This natural variation is potentially useful for quantifying resource use because the isotopic composition of an animal’s diet is assimilated somewhat predictably into the animal’s tissues. In fact, the ratio of naturally-occurring isotopes in animal tissues can be more dependable in dietary assessment than other methods such as stomach, pellet, and fecal analyses and can provide valuable information on the diets of both individuals and populations.

In this study I examined the carbon and nitrogen stable isotope composition of arthropods and fruits on Block Island, Rhode Island. These arthropods and fruits are thought to be important food sources for migratory songbirds that stop on Block Island to rest and replenish fat stores during their southbound fall migration. My objectives were to (1) characterize the isotopic composition of these resources, (2) explore the isotopic differences among various arthropod orders and fruit species, and (3) evaluate potential explanations for any observed differences.

I found significant differences between fruits and arthropods in their nitrogen and carbon stable isotope compositions. I consider this result in the context of using stable isotopes to study the foraging ecology of migratory birds on Block Island. Additionally, I found significant differences in both stable isotopes between several arthropod orders and fruit species and discuss these isotopic differences in relation to their ecology. Determining the extent to which these possible dietary resources differ isotopically is an important first step in evaluating resource use of migratory birds.